Optical beam position active sensing using satellite beams

ABSTRACT

A directed beam position sensing system and method includes a beam source configured to emit a beam. A satellite beam generator is aligned to receive the beam and to transmit a main beam portion and to generate a pair of symmetrically positioned satellite beams. A first detector arrangement is positioned to detect a first satellite beam of the pair and to generate a first detector signal. A second detector arrangement is positioned to detect a second satellite beam of the pair, and to generate a second detector signal, wherein the first detector signal and the second detector signal are used to determine a position of the main beam.  
     In accordance with another aspect of the present application, further provided is a second pair or satellite beams, wherein the first pair of satellite beams is generated at one of an x axis or a y axis, and the second pair of satellite beams is generated in the other axis. The first pair of satellite beams and second pair of satellite beams being perpendicular to each other and including third and fourth detectors to detect the position of the satellite beams.  
     In a further embodiment of the present application, a beam position sensing system includes a beam source configured to generate a directed beam. A satellite beam generator is aligned to receive the directed beam and to transmit a main beam and generate a single-sided satellite beam at a known position. A detector arrangement positioned to detect the satellite beam generates a detector signal. The detector signal is then used to determine the position of the main beam.  
     In accordance with another aspect of the present application, the satellite beam generator is a scanning satellite beam generator which scans the satellite beams, while transmitting a main beam substantially undisturbed.

BACKGROUND OF THE INVENTION

[0001] The present application is directed to beam position detectionand control, and more particularly to systems and methods that enablesensing the position of a beam along one or more axes while minimallyperturbing or interfering with the beam, and, when appropriate, movingof the beam to control its position.

[0002] A number of beam sensing systems and methods are known. One typeof system senses the beam itself. For example, in Bhalla et al., U.S.Pat. No. 6,301,402, the main beam is dithered in order to provideappropriate alignment information and control. However, during sensingof beam position, the beam cannot be used normally for its intendedpurposes. This shortcoming delays or impacts desired operational use ofthe beam.

[0003] Another beam sensing system, U.S. Pat. No. 4,459,690 to Corsoveret al. describes a single dithered beam system, where the output from alight source is split by a beam splitter into a plurality of beams. Thisplurality includes a light beam (PBT) used for tracking a guide track(T) of an optical disc. The tracking is accomplished by dithering thePBT beam that impinges as a light spot (P_(T)) upon guide track (T). Thedithered beam (PBT) is used for tracking purposes in both a playback andrecord mode. In a preferred embodiment, the playback beams are split byan optical grating and dithered by an acousto-optic device. Such asystem generates an error signal which is to be driven to zero, but itdoes not teach measuring the position of a beam over a variable range.Corsover et al., therefore, show a teaching for sensing a relativedistance from a track which is a fixed feature. It does not teachsensing of an actual position of a main beam which permits foradjustable control.

[0004] Accordingly, a new system which provides highly accurate, highlyresponsive position sensing and/or control is desirable. The systemshould provide minimal disturbance of the main beam in its operation,while preferably providing precision sensing and also providing bothrapid response and long-term stability. Such a system would beanticipated to have applicability in a variety of beam sensingapplications, including that for beam position control. Control of abeam in this manner has applicability in a variety of environments,including laser printing, medical laser systems, precision laserinstruments, optical switch networks, free space laser communication,high power lasers, as well as other industrial and academic settings.

SUMMARY OF THE INVENTION

[0005] A directed beam position sensing system and method includes abeam source configured to emit a beam. A satellite beam generator isaligned to receive the beam and to transmit a main beam portion and togenerate a pair of symmetrically positioned satellite beams. A firstdetector arrangement is positioned to detect a first satellite beam ofthe pair and to generate a first detector signal. A second detectorarrangement is positioned to detect a second satellite beam of the pair,and to generate a second detector signal, wherein the first detectorsignal and the second detector signal are used to determine a positionof the main beam.

[0006] In accordance with another aspect of the present application,further provided is a second pair or satellite beams, wherein the firstpair of satellite beams is generated at one of an x axis or a y axis,and the second pair of satellite beams is generated in the other axis.The first pair of satellite beams and second pair of satellite beamsbeing perpendicular to each other and including third and fourthdetectors to detect the position of the satellite beams.

[0007] In a further embodiment of the present application, a beamposition sensing system includes a beam source configured to generate adirected beam. A satellite beam generator is aligned to receive thedirected beam and to transmit a main beam and generate a single-sidedsatellite beam at a known position. A detector arrangement positioned todetect the satellite beam generates a detector signal. The detectorsignal is then used to determine the position of the main beam.

[0008] In accordance with another aspect of the present application, thesatellite beam generator is a scanning satellite beam generator whichscans the satellite beams, while transmitting a main beam substantiallyundisturbed.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009]FIG. 1A illustrates a partial top view of a laser beam sensingarrangement via the use of symmetric satellite beams, wherein thesatellite beams are in an initial state;

[0010]FIG. 1B is an end view of FIG. 1A;

[0011]FIG. 1C illustrates a partial top view of a laser beam sensingarrangement via the use of symmetric satellite beams;

[0012]FIG. 1D depicts an end view of FIG. 1C.

[0013]FIG. 2A sets forth amore detailed partial top view of oneembodiment for a laser beam position sensing via opposite symmetricscanned satellite views with beam detectors;

[0014]FIG. 2B shows a partial end view more particularly illustratingthe detectors of the present embodiment;

[0015]FIG. 3A is a partial top view of a laser beam position sensingsystem for sensing in both the x and y axes using two pairs of satellitebeams generated by two separate beam generators such as acoustic-opticmodulators;

[0016]FIG. 3B sets forth a partial side view of the arrangement shown inFIG. 3A;

[0017]FIG. 3C depicts a partial end view more particularly showing thedetectors implemented in the system shown in FIGS. 3A and 3Brespectively;

[0018]FIG. 4A is a partial top view of a laser beam position sensingsystem implementing two-axis satellite beams, using a single two-axisbeam generator such as a two-axis acousto-optic modulator;

[0019]FIG. 4B is a partial side view of the system shown in FIG. 4A;

[0020]FIG. 4C sets forth a partial end view more specificallyillustrating the detectors for the system of FIG. 4A and FIG. 4B;

[0021]FIG. 5 illustrates an alternative embodiment for detecting thesatellite beams;

[0022]FIG. 6 depicts a block diagram for a laser beam sensing andcontrol system through the use of satellite beams in accordance with thecontext of the present application;

[0023]FIG. 7 shows control loop beam stabilization results for oneembodiment and operation of the present application;

[0024]FIGS. 8A and 8B depict the results of controlled beam steering forthe closed-loop characterization and calibration of the system inaccordance with FIG. 6;

[0025]FIG. 9 sets forth the response versus time for closed loopcorrection and settling when a perturbation is applied to a main beamgenerated in a system such as shown in FIG. 6; and

[0026]FIG. 10 is an expanded scale of the graph of FIG. 9.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0027]FIG. 1A is a top view of beam arrangement 10, such as a laser beamacoustic beam, which includes a main beam 12, and a pair of symmetric,oppositely positioned satellite beams 14 and 16. Each of the satellitebeams 14 and 16 are 1^(st) order diffracted beams, positioned offset byequal distances 20, 22 from an actual position 18 of main beam 12. FIG.1B illustrates a partial end view of beam arrangement 10 where satellitebeams 14, 16 are equally distanced, in opposite directions, from mainbeam 12. In FIGS. 1A and 1B the satellite beams 14, 16 are in an initialbeam position deviation (x₀) from the mainbeam 12. FIGS. 1C and 1D showbeams 14, 16 at a subsequent position deviation (x) from the main beam12. The deviations shown in FIGS. 1A-1D are defined by:

[0028] x=Z tan α(which is similar to x=αZ for small angles), where Z isbeam length and α is a deflection angle.

[0029] Use of beam arrangement 10 enables the sensing of the position ofmain beam 12 along one dimension, while minimally perturbing main beam12. This feature may be applied to a wide variety of beam sensingapplications, particularly for position control.

[0030] As will be explained in more detail in connection with FIGS.2A-2B, beam arrangement 10 permits precision position sensing of mainbeam 12 along one axis or direction (e.g., the x axis) by accuratedetection of the time of crossing for each satellite beam 14, 16 past areferenced position. In one embodiment, the satellite beam referencecrossing is sensed by a differential output bi-cell photo-detector, suchas those which may be used in start of scan (SOS) and end of scan (EOS)detectors in laser printing raster output scanning (ROS) systems. Thesatellite beams can be generated by a variety of satellite beamgenerators such as transmissive or reflective diffraction gratingdevices and/or beam splitters. A preferred generator element is anacousto-optic cell functioning as a diffraction modulator. In someimplementations the satellite beam generators are static elements.However, in preferred embodiments, the satellite beam generators arevariable and act as signal modulators, and the satellite beams arescanned (or dithered). In one instance, an acousto-optic cell scans thesymmetric satellite beams by sweeping the frequency of an RF drive, usedto control operation of the acousto-optic cell.

[0031] Systems built in accordance with the concepts depicted in FIGS.1A-1D cause minimal disturbances to the main beam and its operationwhile providing scanned sensing via the satellite beams. The symmetricdesign enables the establishment of the main beam as centered betweenthe satellite beams, whether their position is stationary or varied.This design enables both accurate position sensing and long-term systemstability.

[0032]FIG. 2A, illustrates a beam position sensing system 30incorporating opposite symmetric satellite beams 14 and 16, positionedan equal distance from main beam 12. In system 30, a beam source 32,such as a laser, emits a beam 34 transmitted to a satellite beamgenerator 36, arranged to diffract the two 1^(st) order satellite beams14 and 16. Main beam 12 is a zero order diffracted beam and is directlytransmitted through satellite beam generator 36. The satellite beams arepreferably a small fraction of the main beam power.

[0033] In a preferred embodiment, satellite beam generator 36 scans eachof the symmetric satellite beams through predetermined paths 38, 40. Asthe satellite beams are scanned through the predetermined paths, mainbeam 12 is maintained substantially unaffected. As mentioned previously,in one implementation, where satellite beam generator 36 is anacousto-optic modulator, the scanning of the beams is achieved bymodulating the frequency of an RF frequency generator 42, providingsignals to the piezoelectric transducer 43 of acousto-optic modulatorsatellite beam generator 36. For typical frequencies, acousto-opticdeflection is a small angle linearly proportional to frequency for agood approximation.

[0034] The deflection angle is given by:

[0035] α=2n arcsin (λf/2nV_(A)) or approximately (λf/V_(A)) for smallangles where,

[0036] λ is the optical wavelength,

[0037] L is the grating thickness (interation length),

[0038] V_(A) is the acoustic velocity,

[0039] Λ is the acoustic wavelength,

[0040] n is the optical index of refraction, and

[0041] f is the acoustic frequency.

[0042] By this design, the deflection provides substantially constantangular velocity for linear frequency sweeps. Consequently, thesatellite beam linear velocity in the axis of the detectors issubstantially a constant magnitude (V) in opposite directions.Optionally the frequency sweep may be adjusted to linearize the scanwith respect to residual active and passive optical scan anglenonlinearity.

[0043] Turning to FIG. 2B, depicted is an end view of system 30, andmore clearly disclosing detectors 44 and 46, which sense the crossing ofsatellite beams 14 and 16 respectively. As noted, in one embodiment,detectors 44, 46 may be differential output bi-cell photodetectors(i.e., differential split detectors), which provide precision crossingtime measurements as sensed from a zero crossing of the differentialoutputs.

[0044] For discussion purposes, main beam position 18 in FIG. 2B is notat system centerline 48. The differential split detectors 44, 46 areseparated a fixed distance 50, 52 (2 d) from the system position 48, andthe distances of the detector midlines 53, 54 from the actual main beamposition 18 are defined as x₁ (56) and x₂ (58). For the arrangementshown in FIG. 2B:

x ₁ =d _(x) +Δx; X ₂ =d _(x) −Δx,

[0045] where Δx (60) is the offset position of the actual main beamposition 18 from the system centerline 48.

[0046] Therefore,

Δx=[x ₁ −x ₂]/2.

[0047] To the approximate linearity of a scan, the scan velocity (V_(x))scales distances to times by:

X ₁ =X ₀ +V _(X) *t _(1x);

X ₂ =X ₀ +V _(x) *t _(2x),

[0048] where t_(1x) and t_(2X) are scan times from start of scan to eachdetector crossing event and x₀ is the symmetric satellite beam positiondeviation from the main beam at the start of scan.

[0049] The offset Δx may therefore be determined by:

ΔX=[t _(1x) −t _(2x) ]*V _(x)/2.

[0050] The satellite beam initial position deviation from the main beamx₀ need not be determined or precisely controlled in this embodimentsince because this variable is eliminated in the symmetric satellitebeam embodiment position determination. Also, it can be shown when thesatellite beam generator is an acousto-optic modulator, the timemeasurements and the relations in the symmetric satellite beam methodabove may be used to determine and calibrate the scan velocity V_(x) aswell as the initial angle deviation x₀, as follows: For an optical pathlength Z and deflection angle from the main beam a, it has been shownthe linear displacement from the main beam is:

x=Z tan αor Z*α, for small α.

[0051] Also, it is recalled from above:

α=2n arcsin (λf/2nV _(A)) or approximately (λf/V _(A)) for small angles.

[0052] Then, for small angles:

x=Z*α=Z(λf/V _(A))=f*(Zλ/V _(A))

[0053] In particular, for the start scan position,

[0054] x₀=f₀*(Zλ/V_(A)) where f₀ is the scan start frequency, and thevelocity is the first time derivative of the position along the scan:

V _(x) =dx/dt=df/dt*(Zλ/V _(A)),

[0055] where df/dt is the time rate of change of the scan drivefrequency which is a constant in the linear scan case.

[0056] Since the sum x₁+x₂ is the constant reference distance measurebetween the two position sensor reference positions, the scanmeasurement equations can be added to give a key result:

X ₁ =X ₀ +V _(X) *t _(1x), and

X ₂ =X ₀ +V _(X) *t _(2x), therefore

X ₁ +X ₂=2d=2*X ₀ +V _(x)*(t₁ +t _(2x))

[0057] The expressions for X₀ and V_(x) in terms of scan frequencies maythen be substituted to give:

2d=2f₀*(Zλ/V _(A))+df/dt*((Zλ/V _(A))*(t _(1x) +t _(2x)),

[0058] which can be rearranged as:

2d=[2f ₀ +df/dt(t _(1x) +t _(2x))]*(Zλ/V _(A))

or

(Zλ/V _(A))=2d/[2f ₀ +df/dt(t _(1x) +t _(2x))]=C.

[0059] Since 2f₀, df/dt, t_(1x) and t_(2x) are precisely measurablefrequency and time variables, the parameter combination C can bedetermined and calibrated.

[0060] Then we have from above:

X ₀ =f ₀*(Zλ/V _(A))f ₀ *C

V _(X) =dx/dt=df/dt*(Zλ/V _(A))=df/dt*C,

[0061] which are likewise precisely calibratable.

[0062] With continuing attention to the symmetric satellite beamembodiments, even if the scan rate of the satellite beam generator 36 isnot linear in time, the offset (Δx) will move to zero when[t_(1x)−t_(2x)] goes to zero in the symmetric deflection scheme of FIGS.2A and 2B. Hence, use of the symmetric satellite beam configurationallows for the design of a robust beam position sensing system capableof providing accurate main beam position data independent of internaland/or external condition variations or fluctuations.

[0063] As will be discussed in greater detail below, the describedsensing operation may be used to control and/or maintain the position ofmain beam 12. In one instance, this position control may be achieved bycalibration of the relation:

X ₁ =g _(1x)(t_(1x);)

X ₂ =g ₂(t_(2x),)

[0064] where g_(1x) and g_(2x) are preferably the same function in thesymmetric scheme here and are the frequencies applied to the satellitebeam generator in the x axis, i.e.:

g_(1x)=g_(2x).

[0065] Therefore,$\Delta_{x} = {\frac{x_{z} - x_{2}}{2} = {\frac{{g_{1x}\left( t_{1x} \right)} - {g_{2x}\left( t_{z\quad x} \right)}}{2}.}}$

[0066] With continuing attention to satellite beam generator 36, it hasbeen mentioned that in one embodiment an acousto-optic cell is used togenerate the satellite beams 14, 16 and to transmit the zero order mainbeam 12. In an acousto-optic cell, an RF signal is applied to apiezo-electric transducer, bonded to a suitableoptically transparentmedium, thereby generating an acoustic wave. This wave induces anoptical phase grating traveling through the medium at the acousticvelocity of the material and with a grating period wavelength dependentupon the acoustic wavelength and hence the frequency of the RF signal.An incident laser beam is diffracted by this grating, which generallymay produce a number of diffracted order beams, depending on thethickness of the grating (or interaction length).

[0067] It is well known that a normalized thickness parameter called theQ determines the number of significant diffraction orders which mayappear. Q is given by:

Q=2πλLf²/n Λ²=2πλLf²/nV_(A),

[0068] where

[0069] λ is the optical wavelength,

[0070] L is the grating thickness (interation length),

[0071] V_(A) is the acoustic velocity,

[0072] Λ is the acoustic wavelength, and

[0073] n is the optical index of refraction, and

[0074] f is the acoustic frequency.

[0075] When Q is much less than 1 (Q<<1), the acousto-optic cell isoperating in what is known as the Raman-Nath mode and there are severaldiffraction orders . . . −2, −1, 0, 1, 2, 3 . . . ) with intensitiesgiven by Bessel functions.

[0076] On the other hand, when the Q factor is much greater than 1(Q>>1), the acousto-optic cell is operating in the Bragg mode. In thisoperation, at one particular incidence angle, only one first diffractionorder is produced. The other orders are suppressed by destructiveinterference. The 1st order beam may be scanned when the frequencycontrolling the acousto-optic cell is varied across an RF bandwidth.

[0077] In the present application, the acousto-optic cell is preferablyoperated in an intermediate regime between a Raman-Nath or Bragg modewhere Q is approximately 1 with approximately normal optical incidenceon the acoustic field and symmetric diffraction into +1 and −1diffracted orders. In this regime the efficiency of both the firstdiffraction orders is optimized while unnecessary higher orders aresuppressed. It is to be understood that an acousto-optic cell operatingin the Raman Nath mode could be used. However, the efficiency of thesystem would be poorer; i.e., more RF power is required for the desireddiffraction efficiency.

[0078] In the experimental implementation undertaken by the inventors,an acousto-optic cell designed for operation in a Bragg mode which wouldnormally be operated in the vicinity of 40 MHz has been used. However,in this implementation, the operational frequency is shifted to below 30MHz, reducing the acousto-optic Q of the cell. In this range, theacousto-optic cell may be operated in an axial mode to produce twosymmetric 1^(st) order beams (i.e., satellite beams 14, 16, scanableover a range of frequencies).

[0079] Further, it is to be mentioned that while in a preferredembodiment the satellite beam generator is described as an acousto-opticcell, beam modulators, variable gratings, beam splitters, vibratingmirrors, optical wedges or other appropriate devices may be used togenerate and/or scan the satellite beams.

[0080] Similarly, while the detectors are shown in a preferredembodiment as differential split detectors, pixel detectors, beamcentroid sensors or other appropriate beam sensors, may be used todetect the satellite beams. Particular types of sensors which may beused are position sensing detectors (PSD), which are photoelectricdevices that convert an incident light spot into continuous positiondata. Two particular position sensing devices are quadrant detectors andlateral effect detectors.

[0081] Turning to FIGS. 3A-3C, illustrated is a laser beam positionsensing system 70 which permits two-axis sensing by use of two satellitebeam generators 72 and 74. The axes may be orthogonal if desired. In topview, FIG. 3A, satellite beam generator 74, is an acousto-optic cellwith transducer 76, and which transmits main beam 12 as the undeflectedzero order diffraction beam and generates symmetric 1^(st) orderdiffracted satellite beams 14 and 16. Acousto-optic cell 72 is alignedto generate satellite beams 14, 16 in a first axis (e.g., the y axis).While this provides a degree of position sensing, it is appreciated thatalso providing position sensing in a second axis (e.g., the x axis)results in additional functionality of two dimensional beam positionsensing, and hence information for two dimensional positioning controlof main beam 12. In side view, FIG. 3B, main beam 12 is still shown asbeing transmitted as a zero order diffracted beam through acousto-opticcell 72. In this design transducer 78 is operated to generate a secondset of satellite beams 80, 82 in the x axis. This two-axis extension ofFIGS. 2A-2B is characteristic in that acousto-optic cells 72 and 74 areboth able to interact with main beam 12 each generating a symmetric pairof first diffraction order beams. Preferably the total portiondiffracted to the satellite beams should be limited to a moderatefraction of the main beam, which may vary depending on theimplementation, but as a general case maybe approximately 10% or less.

[0082] For position detection and control, it maybe desirable that thetwo-axis (x, y) design having the two pairs of satellite beams (80, 82and 14, 16) be perpendicular to each other. However, it is to beunderstood it is not required that the sets of beams be perpendicular toeach other. In these situations, the specific geometry of such a designwill need to be taken into account when determining sensing and controlfunctions.

[0083]FIG. 3C is an end view for two-axis system 70 wherein detectors84, 86 are positioned in the y axis for operation similar to detectors44 and 46 in the x axis. Since the two sets of satellite beams in thedifferent axes are perpendicular to and independent of each other, thecalculations to determine the offset from an ideal centerline positionmay be used for those signals in the y axis as used in the x axis. Moreparticularly the main beam position offset in the x and y axes aredetermined by:

Δx=[t _(1x) −t _(2x) ]*V _(x)/2; and

Δy=[t _(1y) −t _(2y) ]*V _(x)/2.

[0084] Similar to the embodiment of FIGS. 2A, 2B, the two sets ofsatellite beams are scanned by operation of the frequency supplied tothe respective acousto-optic cells. The relationships between elementssubstantially are the same in the y axis as those discussed in relationto the x axis. Particularly, detectors 84 and 86 are positioned an equaldistance from the system (y axis) centerline 90. When the actual system(y axis) centerline 92 is shown in this example to be a distance fromthe ideal main beam (y axis) centerline 90, the difference is the offset(Δy) 93 in the y axis. This geometric positioning results in similarequations being used both in the x and y axes. Thus, the distances ofthe midlines of the detectors from the actual main beam centerline inthe y axis are y₁ and y₂, with:

y ₁ =d _(y)+Δ_(y) ; y ₂ =d _(y−Δ) _(y)

[0085] Δy is the offset of the position of the main beam from themidline between the two split detectors. Therefore:

Δy=[t _(1y) −t _(2y) ]*V _(x)/2.

[0086] To obtain approximation of a linearity of scan, the scannedvelocity scales distances to times by:

y ₁ =V _(y) ×t _(ty) ; y ₂ =V _(y) *t _(2y).

[0087] Therefore, offset in the y axis is:

Δy=[t _(1y) −t _(2y) ]*V _(x)/2.

[0088] It is to be appreciated, that even if the scan is nonlinear intime, the offset will go to zero when [t _(1y) −t _(2y)] goes to zerodue to the symmetric deflection scheme arrangement. Thus, robustcentering of the y axis is possible independent of internal and/orexternal condition variations. If it is desired to use this sensingoperation to control and/or maintain the position of main beam 12 in they axis, correction operations can be employed. In general, all that isneeded is calibration information on the relation, specifically,

y ₁ =g _(1y)(t _(1y)); Y₂=g_(2y)(t_(2y)),

[0089] where g_(1y) and g_(2y) are the frequencies applied to thesatellite beam generator in the y axis.

[0090] Since the frequency should be the same in the present symmetricscheme, then:

g _(1y) =g _(2y).

[0091] Thus, the x and y satellite beam sense systems may be operatedindependently of each other, as well as simultaneously to each other. Onthe other hand, with existing beam sensing and control methods whichprovide dithered sensing of the main beam itself, two dimensionaldithered sensing is generally not workable.

[0092] Turning to FIGS. 4A-4C, a further embodiment of a two-axissatellite beam system 94 using a two-axis beam generator 95, such as atwo-axis acousto-optic modulator cell with transducers 96 a, 96 b, isillustrated. In this design, the acousto-optic cells are integrated on asingle substrate. This configuration has similar operational concepts asFIGS. 3A-3C. However, due to the use of an integrated two-axisacousto-optic cell design, the embodiment of FIGS. 4A-4C has two feweroptical surfaces. On the other hand, the simultaneous adjustment of thetwo modulators' angular alignment with less adjustment degrees offreedom may be less convenient, although angle tolerance should not becritical in the lower Q axial mode. Finally, the overlapping integratedstructure may enhance second order spurious cross-diffracted beams whenboth axes are operating simultaneously. This is true since both soundbeams are interacting with both sets of 1^(st) order diffractionsimultaneously in the same volume. It is estimated that if the two axeshave equal diffraction efficiencies, then the second order spuriouscross beams should be about four times more intense than they would bein the separate configuration of FIGS. 3A-3C. This effect can also beprevented by separating the interactions with transducers displacedalong the optical beam path, resulting in a single thicker two axisacousto-optic cell. Applicants note that detectors 44 and 46 are shownsized differently than detectors 84 and 86. In other embodiments, thedetectors may be sized and configured the same. However, FIGS. 3C and 4Cillustrate that this is not a requirement.

[0093] The foregoing embodiments depict the satellite beams as beingemitted and detected in the same physical space as the main beam.However, this is not a requirement, rather deflection of the satellitebeams may be undertaken to direct the satellite beam as desired. Forexample, in system 97 of FIG. 5, mirrors 98 and 99 are arranged todeflect satellite beams 14 and 16 to detectors 44 and 46, respectivelypositioned at alternative locations from the previous embodiments. Thisdesign is provided to emphasize it is not necessary for the satellitebeams to be detected in the same space or plane as the main beam. Forexample, in embodiments where the main beam, may enter a destructiveenvironment or isolated location, it may not be desirable to include thedetectors in that environment or location. Such re-positioning may alsobe useful if the location imposes size constraints making it difficultto incorporate the beam detectors. However, in this embodiment thedeflecting mirrors and beam detectors will require superior mechanicalstability.

[0094] The foregoing discussion primarily focuses on a symmetric activesensing system, taking advantage of the generation of a pair or a set ofpairs of symmetric satellite beams positioned in relationship to a mainbeam. Itis possible, however, to employ an asymmetric system using asingle-sided satellite beam to sense main beam position data when theinitial scan position and scan velocity can be accurately determined.

[0095] Particularly, when the beam generating device is an acousto-opticcell, operation of the cell in a Bragg mode results in a singlesatellite beam. It is to be understood other types of beam generatorsmay be used to generate a single-sided satellite beam output. Therefore,in the system shown in FIGS. 2A and 2B, for example, one of thesatellite beams 14 and 16 is eliminated. The corresponding detectors 44or 46 will also be unnecessary. In such a design, by knowing the initialscan position and the scan velocity, the main beam position is obtainedthrough the use of a single-sided satellite beam. For example, similarto one beam of the two-beam system, to approximate the linearity of ascan, the scan velocity (V_(x)) scales distances to times by:

x ₁ =x ₀ +V _(x) *t _(1x),

[0096] where t_(1x) is the scan time from a start of scan for thedetector crossing event, x₀ is the single-sided satellite beam positiondeviation from the main beam at the start of scan and v_(x) is the scanvelocity. In this case x₁ is the determined distance from the singlesensor, which is the meaningful output of the single-sided system.

[0097] Additionally, the systems illustrated in FIGS. 3A-3C, 4A-4C, 5and the system of FIG. 6 may also operate in this single-sided satellitebeam mode. With particular attention to FIGS. 3A-3C and FIGS. 4A-4C(where pairs of satellite beams are provided in both the x and y axes)beam generators may be operated to generate the single-sided beam ineach of the x and y axes. Thus, single-sided beam position sensingaccomplished via the single-sided embodiments in both the x and ypositions.

[0098] The initial beam position is obtained by a variety of knownmethods, including an initial calibration through the use of externalmechanical and/or electronic measuring devices, as is known in the art.It is also to be appreciated that, as in the two-sided embodiments thedetectors, though described as primarily split detectors, may also berepresentative of position sensing detectors (PSD), which provide acontinuous position signal of the single-sided satellite beam.

[0099] Turning to FIG. 6, shown is a block diagram for a laser beamsensing and control system 100 which implements the described positionsensing satellite beam concepts. While for convenience FIG. 6 showssatellite beams in a single axis, it is to be recognized system 100 maybe used in connection with the dual satellite beam systems havingsatellite beams existing in the x and y axes as previously described.

[0100] Control system 100 may implement a variety of control schemes tocause the main beam to be steered to a desired alignment. A particularscheme is known as proportional integral derivative (PID) control. InPID control, the corrective action uses the proportional part of thecontrol dependant upon the magnitude and sign of the error signal. Thetime integral of the error, or the magnitude of the error multiplied bythe time that it has persisted, is addressed by the integral portion ofthe scheme, and the time rate of change of the error (i.e., a rapidlychanging error causes a greater corrective action than a slowly changingerror) is addressed by the derivative part of the control.

[0101] In an intuitive sense, the derivative part of the controllerattempts to look ahead and foresee if the process is in for a largerchange than might be expected based on present measurements. That is, ifthe measured variable is changing very rapidly, it is likely that it isgoing to try to change by a large amount. This being the case, thecontroller attempts to predict the value of the change and applies amore corrective action than would be initially considered appropriate.

[0102] It is to be appreciated that while a PID controlled scheme isdescribed as a preferred embodiment, other control systems may alsoused, including proportional integral (PI) control or any other schemewhich would use the sensed position data to maintain or re-position themain beam.

[0103] Control system 100 enables determining a position of a main beamvia sensing the satellite beams while minimally perturbing the main beamitselfand further includes, when needed, a re-positioning of the mainbeam, by actuating closed loop control, based on sensed positions of thesatellite beams.

[0104] With continuing attention to FIG. 6, a beam source 102, such as alaser, emits a beam 104 received by a beam deflector 106. The beamdeflector 106 may be a TeO₂ acousto-optic modulator operating, forexample, in a Bragg mode, although other beam deflectors may also beused such as one of many types of controllable mirrors. In thisembodiment the acousto-optic deflected beam 108 is passed to a satellitebeam generator 110, configured as a previously disclosed acousto-opticcell or other appropriate satellite beam generation device. Satellitebeam generator 110 is supplied with a signal from RF sweep signalgenerator 112 amplified by RF amplifier 114. The sweep signal scans ordithers satellite beams 116 and 118. Main beam portion 120 passesthrough satellite beam generator 110 substantially undisturbed to target122.

[0105] In this embodiment, the acousto-optic deflected beam 108 ispassed to a satellite beam generator 110, which is configured as apreviously disclosed acousto-optic cell or other appropriate satellitebeam generation device. Satellite beam generator 110 is supplied with asignal from RF sweep signal generator 112 amplified by RF amplifier 114.The RF signal scans or dithers satellite beams 116 and 118. Main beamportion 120 passes through satellite beam generator 110 substantiallyundisturbed to a target 122, such as a beam profiler.

[0106] Main beam portion 120 is intended to be positioned at a specificlocation on target 122. However, it is understood that due to externalinterferences, imperfections in the system components or for otherreasons, main beam portion 120 may not align to an ideal position.Control system 100 is therefore designed to not only sense a position ofthe main beam portion 120, but to also, if needed, alter thelocation/alignment of main beam portion 120. Control system 100 firstdetermines the position of the main beam portion as previouslydescribed, and then acts to re-align the main beam portion, when needed,to obtain an accurate (desired) alignment.

[0107] The position sensing operation occurs as previously describedwhere satellite beams 116 and 118 are scanned or dithered acrossdetectors 124, 126, are such as differential split detectors, to acquiredata needed for position sensing. Detectors 124 and 126 provided withzero-crossing detection circuits to generate output temporal signals128, 130, where in this embodiment the output signals are rectangularpulses with edges occurring at times corresponding to the time ofcrossing of the satellite beams (116, 118) past the split detectorcenter lines, respectively. As shown in this example, edges of signal128 are shifted from the edge timing of signal 130. This information isused to determine the position of the main beam portion a and any offsetfrom the desired main beam portion position. The amount and direction ofthe offset (Δx) is determined by reference to the previously recitedrelationships. Control system 100 uses the obtained position informationto provide feedback control for a control scheme, such as PID control,to alter the position of 1^(st) order deflected beam 108 and in turnmain beam portion 120.

[0108] Specifically, output signals 128 and 130 from detectors 124 and126 are supplied to a time difference computation block 132, whichdetermines a time difference between satellite beams 116, 118 as theyare scanned to detectors 124 and 126.

[0109] The time difference calculated by computation block 132 may, inthis embodiment, be the time difference (Δt) 133 between signals 128 and130, where having Δt at zero indicates the main beam portion 120 is atthe nominal centerline position. The computation of this time delay isundertaken by the previously described relationships. Computation block132 may be part of a computer or may be a separate computational device.In embodiments when other detectors are used, for example, when PSDdetectors 44, 46, 84, 86 are used, position detection of the beams maybe continuously and directly obtained.

[0110] The time difference information or data is provided to acontroller such as control block 134 which incorporates a known controlscheme, such as PID or other appropriate control. Control block 134 maybe part of the computer system, or alternatively, data from computationblock 132 is provided to control block 134 via an interface such as aPCI interface. Control block 134 takes the time differential information(Δt), which may be provided as a continuous stream of data, or as datablocks, and generates a compensation signal designed to alter the outputof RF generator 136. The output of the RF generator is adjusted byattenuator 138, amplified by amplifier 140 and then supplied toacousto-optic deflector 106. When corrective movement of beam 108 isrequired, based on the time difference data, control block 134 instructsRF generator 136 to generate a signal having a frequency different froman existing signal frequency being supplied to acousto-optic deflector106. Altering the frequency of the signal supplied to the acousto-opticcell 106 alters the deflection angle of the 1^(st) order diffracted beam108, as detailed previously. In other embodiments appropriate beamdeflection signals would be generated; for example, a mirror tiltadjustment signal.

[0111] Hence, altering the RF frequency moves the 1^(st) order beam 108generated at acousto-optic cell 106. The degree of movement isdetermined by the frequency supplied which in turn is dependant on theoutput from detectors 124 and 126. An RF spectrum analyzer 142 and powermeter 144 are provided to monitor the signal operation of control system100 in a laboratory implementation.

[0112] The time difference between the signals from detectors 124 and126 is used to control the output of RF generator 136 to steer the mainbeam portion 120 to a desired position. This position is in oneembodiment based on a calibrated control portionality constant foreffective servo control. For critically damped control, the beam maybepositioned within noise limits (+/−{fraction (1/60)} beamdiameter—experimentally realized) based on one pair of zero crossingreadings.

[0113] With continuing attention to FIG. 6, a main beam, such as mainbeam portion 120, may not be located at its ideal position due toperturbations inserted into the system. These can occur due to changeswithin the system, such as individual components not operating withinexpected parameters, or it may be due to external environmentalconditions or actions (i.e., a bumping or movement of the system). Lines146 are used to designate perturbations (both internal and external).Particularly, lines 146 represent an external jarring of the system aswell as other external or internal beam position altering factors.Control system 100 is therefore designed to move the main beam portion120 to a desired location in response to the occurrence of such internalor external perturbations. It is to be noted that system 100 may bedesigned whereby the nominal centered position and the desired positionare not necessarily the same location. As has been used herein, thenominal centered position may be a location where zero time differenceoffset signal exists. However, in some situations a user may wish thatthe main beam portion be located at a position offset somewhat from thenominal center position. This may be accomplished simply by selection ofappropriate parameters for the controller to provide a desired offset,which would be well known to one of ordinary skill in the art.

[0114] It is to be noted the control system described therein may beused with each of the sensing systems described throughout FIGS. 1A-5,and the corresponding single-sided satellite beam sensing systems. Inthat regard, for the embodiments using sensing in both the x and y axes,for both the two-sided embodiments and the single-sided embodiments,main beam movement will occur in both the x and y axes. To accomplishthis task, in one embodiment, and as previously mentioned, the beamdeflector 106 may be one of many types of angle-controllable mirrorswhich allow movement in the x and y axes. Alternatively, acousto-opticcells such as those shown in FIGS. 3A, 3B, 4A and 4B may also beimplemented in the Bragg mode. By this design, operation of theappropriate acousto-optic cell for movement in the corresponding axis,i.e., the x or y axis, is undertaken to move the 1^(st) order beam 108.In this embodiment, there would be additional output signalscorresponding to output signals 128 and 130 from the additionaldetectors such as detectors 124 and 126. The time difference computationblock 132 is then simply configured to receive the signals, and usingpreviously described processes, determine time differences for detectionin the both the x and y axes. Thereafter, the controller, such ascontrol block 134, is designed to provide corrective signals to an RFgenerator 136. In this embodiment, the RF generator is connected to beamdeflectors for operation in the x and y axes, and provides an outputsignal through the attenuator 138 and amplifier 140 to such a beamdeflector 106.

[0115] Beam actuation or steering of a closed-loop control is providedin existing systems by the use of mechanical devices such as therotating or movable mirrors. The position sensing approaches and systemsdescribed in the foregoing discussion is applicable to such systems.Further, the acousto-optic actuation or steering approaches and systemsimplemented in the described control systems may also be implementedseparate from the sensing operations described herein. Particularly, thebeam actuation systems may be used in a variety of implementations wherehigh-speed free response is of value.

[0116] An attribute of control system 100 is the efficiency and speedwith which the system returns the main beam portion to its intendedposition. FIG. 7 is a chart 150 demonstrating the operational capabilityof the control system 100 of FIG. 6.

[0117] Chart 150 compares the beam position versus the perturbationfrequency. In experimental operation of control system 100, theinventors intentionally inserted an increase in frequency which alteredthe output from beam deflector 106. The desired operational position isat slightly above 6100 μm when there is a frequency of approximately 50MHz. When no feedback control is provided and a perturbation offset isintroduced, the position of the main beam is not maintained at aconsistent location as shown by no-control points 152. Rather, alteringof the deflector drive frequency moves the beam position. However, whencontrol circuitry of FIG. 6 is implemented, the control systemcompensates for the generated offset which occurs due to theperturbation, as shown by control points 154. In experimentalobservations, a +/−{fraction (1/60)} beam width feedback control jitterwas achieved on a 1 mrad angular divergence optical laser beam over 3.5mrad position adjustment range. This was obtained with off-the-shelfcomponents, and improved results would be obtained by using componentsspecifically designed for the systems of the present application.

[0118] Turning to FIGS. 8A and 8B, set forth are results of experimentsusing a control system such as described in connection with FIG. 6.Chart 156 of FIG. 8A plots the frequency supplied to beam deflector 106versus the time difference in the detected signals generated from thesatellite beams. The changing of the frequency occurs during a beamsteering operation. Plot line 158 emphasizes a characteristic of whathappens when the frequency keeps changing the beam position. The timedifference exists as the satellite beams continue to hit the sensors atdifferent points. Chart 160 of FIG. 8B shows the beam position versusthe time difference as plot line 162 which emphasizes the changing beamposition. These charts reflect the experimental performance of thelinear deflection positioning, time difference signals and beamdeflector drive frequency.

[0119] Chart 164 of FIG. 9 shows the response transient time to returnthe detector crossing time difference sensing signal to a desired level,when, for example, a perturbation of main optical beam position causes aperturbation time difference 166, to deviate from the desired value.FIG. 10 is an expanded scale of the time response of FIG. 9. Forexample, when perturbation is applied to the system (in an experimentaloperation), perturbation time difference change 166 is shown to occur(e.g., from 0.007 sec to 0.005 sec—in this case the perturbationdecreases the time difference, which means the system had an intended Δtof 0.007). However, using the active closed loop control of FIG. 6, thesystem adjusts to the desired offset time difference (e.g., 0.007 sec).Particularly, as shown in FIG. 10, the time difference in seconds isnormally 0.007 seconds. At a time of approximately 4180 msec, theperturbation causes the perturbation time difference 166. During asatellite beam scan cycle (i.e., within about 20 msec in this laboratoryimplementation), the operational control of FIG. 6 settles control backto the desired time difference (i.e., the beam is directed back to itsintended alignment position). It is to be understood, the 20 msec cycletime in this case was limited by the particular control electronic used.Ultimately, the scan cycle time is limited by the satellite beam scannercapability, which is typically in the microsecond range for acoustoopticmodulators, limited by the transit time of the acoustic wave across theoptical beam.

[0120] As previously mentioned, the inventors have undertakenexperimental operations using a system as shown in FIG. 6. Listed belowfor single-beam fine angles is a summary of various specifications whichhave been achieved and those that are considered realizable for systemsemploying the described methods. It is to be understood this chart isnot intended to limit the realizable ranges of systems configured inaccordance with the disclosed methods. Realizable Ranges of Parameterfor Specs: Demo Method Described Position Sensing Range 6 beam widths 2to 100 3.4 mrad Position Sensing Precision .015 beam width. .02 mradPosition Sensing Accuracy .015 beam width .w. similar [Est] PositionSensing Updates 20 msec .01 to 1 msec (bus limited) Feedback Settling-AOscanner 80 msec 0.01 msec Feedback Settling-mirror 1-10 msec Wavelength632.8 nm 300-10000 nm [detector material] Optical Insertion Loss [Off]<0.05 dB Optical Insertion Loss [On] 1 dB nominal <0.2 dB 2D XY sensing[dual system] similar to 1D Min Optical satellite Beam 0.1 mw 0.01 mw[Est-TBD] Power Max optical beam power approx. est 5 watts or more 5miliwatts

[0121] The above-described systems and methods provide beam positionsensing which has a minimal impact on a main beam or its use. Also, theuse of satellite beams avoids excess power on the position sensors whenthe main beam carries high optical power. The system provides a highlyprecise and accurate beam sensing operation, which maybe <{fraction(1/60)}beam diameter or {fraction (1/60)}millirandian. The describedsystems and methods also provide for a fast sensing down to the order ofmicrosecond scan times through the use of a fast satellite beamgenerator such as the acousto-optic cell. Use of scan or ditheredsensing operations permit for a robustness with respect to noise andlong-term drift. The symmetric configuration of the satellite beamscancels out potential uncertainties, systematic errors and drifts,present in other systems such as asymmetric systems. The systemconfiguration permits calibration stability and monitoring of thecalibration during operation of the sensing system.

[0122] In addition to the foregoing, the two-axis system providesindependent sensing of position along two non-collinear axes. Further,the operations on the two axes may be considered to by synergistic.Particularly, two sensing operations can be accomplished simultaneouslyor sequentially, in phase or out of phase, synchronous or asynchronous.The two-axis system maybe beneficial in the use of sense operations andcontrol operations in a two-axis mirror system by alternating the senseand adjustment on the two axes.

[0123] Implementing the concepts in control systems such as described,takes advantage of using generation and position sensing of thesymmetric scanned satellite beams to precisely and robustly sense theposition of a main beam, in combination with an active closed-loopcontrol actuation to position the main beam. It is to be appreciated,and as has been described previously, that the beam may be controlled tobe centered at a nominal centerline position, or controllably offset bya desired amount. The two beam approach combines the symmetric beamsensing with the, acousto-optic deflector to reposition the main beamportion in a fast, non-interfering manner. Further, the combination ofthe symmetric sensing operations to the calibrated control deflectionalso provides fast, servo convergence of the position sensing operationand control processes.

[0124] It is to be understood that many alterations to the systems andmethods described herein may be appreciated by one upon reading andunderstanding of the present description. It is understood that allalterations and modifications to the foregoing described systems andmethods may be undertaken without departing from the spirit and scope ofthe invention as determined by the following claims and theirequivalents.

What is claimed is:
 1. A beam position sensing system comprising: a beamsource configured to generate a directed beam; a satellite beamgenerator aligned to receive the directed beam and to transmit a mainbeam and generate a pair of symmetric satellite beams; a first detectorarrangement positioned to detect a first satellite beam of the pair ofsatellite beams and generate a first detector signal; and a seconddetector arrangement positioned to detect a second satellite beam of thepair of satellite beams and generate a second detector signal, whereinthe first detector signal and the second detector signal are used todetermine a position of the main beam.
 2. The invention according toclaim 1, wherein the symmetric satellite beams are oppositely positionedfrom each other and are an equally spaced linearly increasing distancefrom the main beam.
 3. The invention according to claim 1, wherein thesatellite beam generator is a scanning beam generator which scans thefirst satellite beam and the second satellite beam, while transmittingthe main beam in a substantially, undisturbed direction.
 4. Theinvention according to claim 1, wherein the first detector arrangementis a set distance in a first direction from a centerline, and the seconddetector arrangement is located a set distance in a second directionfrom the centerline, each detector arrangement positioned to detect oneof the satellite beams of the pair.
 5. The invention according to claim4, wherein the main beam is offset (Δx) from the centerline.
 6. Theinvention according to claim 5, wherein the offset (Δx) value of themain beam is found by: Δx=[t ₁−t₂ ]*V/2, wherein t₁ is a time one of thefirst detector system and the second detector system is scanned, t₂ is atime the other one of the first detector system and the second detectorsystem is scanned, and V is the velocity at which the scan of the firstand second detector arrangements occurs, which are the same.
 7. Theinvention according to claim 1, wherein the scanning of the firstsatellite beam and the scanning of the second satellite beam begin atsubstantially the same time and detection of the first satellite beam bythe first detector arrangement and detection of the second satellitebeam by the second detector arrangement occurs at different times,determining the main beam position.
 8. The invention according to claim7, wherein the main beam is minimally disturbed in magnitude anddirection.
 9. The invention according to claim 1, wherein the satellitebeam generator is an acousto-optic modulator.
 10. The inventionaccording to claim 1, wherein the satellite beam generator is anacousto-optic modulator with Q<1.
 11. The invention according to claim1, wherein the satellite beam generator is an acousto-optic modulatorwith a Q approximately equal to
 1. 12. The invention according to claim1, wherein deflection of the first and second satellite beams are atangles linearly proportional to a sweep frequency supplied to thesatellite beam generator.
 13. The invention according to claim 1,wherein the first and second satellite beams each have a linear velocityin the plane of the detector arrangements which are of a substantiallyequal constant magnitude in opposite directions to each other.
 14. Theinvention according to claim 3, wherein a scan position versus time isnonlinear.
 15. The invention according to claim 9, wherein theacousto-optic modulator is operated in an axial mode with a low enoughacousto-optic Q to a level to produce the pair of symmetric satellitebeams.
 16. The invention according to claim 9, wherein the acousto-opticmodulator is operated in an axial mode with an acousto-optic Q ofapproximately
 1. 17. The invention according to claim 1, wherein thefirst detector arrangement and the second detector arrangement are eachone of differential split detectors or position sensing detectors. 18.The invention according to claim 1, wherein the pair of symmetricsatellite beams are a first pair of satellite beams generated at one ofan x axis or a y axis at an angle to each other.
 19. The inventionaccording to claim 18, further including a second pair of satellitebeams including a first satellite beam of the second pair and a secondsatellite beam of the second pair, the second pair of satellite beams inone of an x axis or a y axis at an angle to each other.
 20. Theinvention according to claim 19, wherein the first pair of satellitebeams and the second pair of satellite beams are perpendicular to eachother.
 21. The invention according to claim 19, further including athird detector arrangement and a fourth detector arrangement, eachpositioned to detect one of the satellite beams of the second pair ofsatellite beams.
 22. A method of sensing a position of a beam emitted:aligning a satellite beam generator to receive the directed beam;receiving the directed beam by the satellite beam generator; generatingby the satellite beam generator a pair of symmetric satellite beams;transmitting through the satellite beam generator a main beam; aligninga first detector arrangement to receive a first satellite beam of thepair of satellite beams; aligning a second detector arrangement toreceive a second satellite beam of the pair of satellite beams, whereinthe first and second detector arrangements are separated a fixeddistance; sensing, by the first detector arrangement, the firstsatellite beam; sensing, by the second detector arrangement, the secondsatellite beam; and determining a position of the main beam based on thesensed the first satellite beam and the sensed second satellite beam.23. The method according to claim 22, wherein the aligning of the firstdetector arrangement and the second detector arrangement includes,determining a centerline for referencing of the main beam; positioningthe first detector arrangement a set distance from the centerline in afirst direction, and positioning the second detector arrangement a setdistance from the centerline in a second direction.
 24. The methodaccording to claim 22, further including scanning the first satellitebeam and the second satellite beam, while transmitting the main beam ina substantially undisturbed direction.
 25. The method according to claim24, wherein the scan of the first satellite beam and the scan of thesecond satellite beam have a substantially equal velocity magnitude. 26.The method according to claim 24, wherein the step of generating thepair of satellite beams includes deflecting the first and secondsatellite beams at angles linearly proportional to a sweep frequencysupplied to the satellite beam generator.
 27. The method according toclaim 24, wherein the first and second satellite beams each have alinear velocity in a plane of the detector arrangements which aresubstantially equal magnitude in opposite directions to each other. 28.The method according to claim 24, wherein the scan velocity isnonlinear.
 29. The method according to claim 24, wherein the satellitebeam generator is an acousto-optic modulator, and further includingoperating the acousto-optic modulator in an axial mode with an effectiveacousto-optic Q low enough produce the pair of satellite beams.
 30. Themethod according to claim 24, wherein the satellite beam generator is anacousto-optic modulator, and further including operating theacousto-optic modulator in an axial mode with an effective acousto-opticQ of approximately
 1. 31. The method according to claim 22, wherein thepair of symmetric satellite beams are a first pair of satellite beamsgenerated in one of an x axis or a y axis at an angle to each other. 32.The method according to claim 31, further including a second pair ofsatellite beams including a first satellite beam of the second pair anda second satellite beam of the second pair, the second pair of satellitebeams in one of an x axis or a y axis at an angle other than the axis ofthe first pair of satellite beams.
 33. The method according to claim 31,wherein the first pair of satellite beams and the second pair ofsatellite beams are perpendicular to each other.
 34. The methodaccording to claim 32, further including a third detector arrangementand a fourth detector arrangement, each positioned to detect one of thesatellite beams of the second pair of satellite beams.
 35. The methodaccording to claim 21, wherein the method of generating the pair ofsymmetric satellite beams is accomplished by an acousto-optic modulator.36. The method according to claim 35, further including: determining ascan velocity (V_(x)) for at least one of the satellite beams; andcalibrating the scan velocity (V_(x)) for at least one of the satellitebeams, by determining the distance between the detectors, measuring aprecisely measurable frequency variable, and measuring a preciselymeasurable time variable.
 37. The method according to claim 35, furtherincluding: determining an initial beam position deviation (x₀) of atleast one of the satellite beams from the main beam; and calibrating theat least one initial beam position deviation of the at least onesatellite beam, determining the distance between the detectors,measuring a precisely measurable frequency variable, and measuring aprecisely measurable time variable.
 38. A beam position sensing systemcomprising: a beam source configured to generate a directed beam; asatellite beam generator aligned to receive the directed beam and totransmit a main beam and generate a single-sided satellite beam at aknown position; and a detector arrangement positioned to detect thesatellite beam and generate a detector signal, the detector signal isused to determine a position of the main beam.
 39. The inventionaccording to claim 38, wherein the satellite beam is an acousto-opticcell operating in a Bragg mode.
 40. The invention according to claim 38,wherein the satellite beam is positioned a spaced linearly increasingdistance from the main beam.
 41. The invention according to claim 38,wherein the satellite beam generator is a scanning beam generator whichscans the satellite beam at a known velocity, while transmitting themain beam in a substantially, undisturbed direction.
 42. The inventionaccording to claim 38, wherein the detector arrangement is a setdistance from areference line, the detector arrangement positioned todetect the satellite beam.
 43. The invention according to claim 38,wherein the detector arrangement is one of a differential split detectoror a position sensing detector.
 44. A method of sensing a position of amain beam: aligning a satellite beam generator to receive a directedbeam; receiving the directed beam by the satellite beam generator;generating by the satellite beam generator a single-sided satellite beamat a known position; transmitting through the satellite beam generatorthe main beam; aligning a detector arrangement to receive a satellitebeam; sensing, by the detector arrangement, the satellite beam; anddetermining a position of the main beam based on the sensed satellitebeam.
 45. The method according to claim 44, wherein the generating stepincludes operating an acousto-optic cell in a Bragg mode.
 46. The methodaccording to claim 44, wherein the aligning of the detector arrangementincludes, determining a reference line for a location of the main beam;and positioning the detector arrangement a set distance from thereference line in a first direction.
 47. The method according to claim44, further including scanning the satellite beam, while transmittingthe main beam in a substantially undisturbed direction.
 48. The methodaccording to claim 44, wherein the scanning step includes scanning at aknown velocity.